The invention relates to caps for writing instruments, e.g., caps for ball point pens.
Writing instruments that contain liquid ink often include caps to prevent the ink from drying out. Such caps are sometimes provided with vent holes at the closed end. These vent holes reduce the pressure exerted on the ink within the writing instrument when the cap is applied, reducing the likelihood that ink will leak from the pen due to the pressure differential caused by applying and removing the cap in a normal manner. Also, if the cap is swallowed by a small child the vent holes may allow the child to breathe with the cap lodged in the child's throat. These vent holes may allow air to flow past the pen tip and cause the ink at the tip to dry out.
One problem that is sometimes encountered with pen caps is "cap pump". This is a phenomenon that occurs when a user of the pen repeatedly pumps the cap on and off of the pen tip. This pumping forces air into the pen through an air vent that is normally provided on the side of the pen tip. The air then travels into the ink reservoir of the pen, displacing ink which then travels up to the tip and out through the air vent. Thus, ink may undesirably "burp" out of the tip and into the cap.